Unexplained high anion gap metabolic acidosis: the golden tipAn 85-year-old woman was admitted to the intensive care unit for monitoring because of underlying heart problems. Initially, she was admitted to the hospital due to an infected knee prosthesis. She had already received intravenous flucloxacillin for several weeks when she developed decompensated heart failure. She was awaiting a coronary bypass when an unusual cause of high anion gap meta­bolic acidosis was diagnosed. The reason for this unusual metabolic acidosis appeared to be the accumulation of 5-oxoproline or pyroglutamic acid. The cause of this accumulation turned out to be the intravenous flucloxacillin in combination with the chronic ingestion of acetaminophen in the context of other common risk factors in an intensive care unit, such as malnutrition and severe illness.